Response:

We would not expect the evolution of photosynthesis to be easy to
unravel. Photosynthesis has been evolving for more than three billion
years, originating even before eukaryotes (Awramik 1992). Its early
history involved gene transfer among several phyla of bacteria, making
the trail harder to trace genetically (Raymond et al. 2003). Different
components of photosynthesis have independent evolutionary pathways.
However, much progress has been made in determining how photosynthesis
evolved (Baymann et al. 2001; Blankenship 1992; Blankenship and
Hartman 1998; Xiong and Bauer 2002).